1. Field of the Invention
The present invention relates to a method and an apparatus for recording information on an optical recording medium used in writing fine record marks and space trains in the so-called ultra high density optical recording and reproducing system.
2. Description of the Related Art
High density optical recording and reproducing media using a blue-violet laser such as BD (Blu-ray Disc) and HD-DVD (High Density Digital Versatile Disc) systems are proposed and partly manufactured in recent years. In these media, the wavelength of a laser beam for recording and reproducing is shortened and the NA (numerical aperture) of an optical system is increased as compared with a conventional optical recording and reproducing system in order to increase storage capacity. Also shortening a minimum mark length in a modulation signal relatively to the diameter of a laser spot furthermore increases areal density. In these examples, the minimum mark length does not exceed the resolution limit of the optical system, though the minimum mark length is shortened relatively to the diameter of the laser spot. Fully using PRML (Partial Response Maximum Likelihood), however, a system in which a minimum mark length is almost equal to the resolution limit is proposed in a conventional medium. Furthermore, as described in Japanese Patent Laid-Open Publication No. 2003-6872, for example, a super-resolution optical recording and reproducing medium which is configured to be able to reproduce a minimum mark length exceeding the resolution limit of a reproduction optical system and a super resolution recording and reproducing system using the medium are proposed.
In the foregoing optical recording medium, it is possible to reproduce fine marks and blank trains by an advanced signal processing method such as PRML or by means of super-resolution reproducing power on the side of the recording medium. However, it is anyway necessary to precisely write the fine marks and blank trains correctly in the direction of a time base and the direction of a reflection level before reproduction.
The conventional optical recording medium adopts the so-called heat mode writing, by which the energy of a laser beam is converted to generate heat in writing. Thus, in writing record marks, the smaller the minimum marks and blanks are set relatively to the laser spot, the more effects an ambient thermal environment has. The ambient thermal environment is thermal effects, for example, occurring in marks and space trains before and after target mark (data). To compensate these effects, various write strategies are proposed since before.
In DVDs (MML/RL=400 nm/270 nm=1.48), for example, MML (minimum mark length) is closer to RL (resolution limit) as compared with CDs (compact disc; MML/RL=840 nm/433 nm=1.94), so that a method called recording compensation (adaptive control) is adopted.
The recording compensation is a method by which the length of a write pulse for writing a record mark is delicately controlled in accordance with a blank length just before the record mark even if the write pulses have an equal NRZI (non return to zero inverted) data length.
When the blank lengths just before a 3T mark (T represents a channel bit length), a minimum mark, are 3T and 6T, for example, the pulse length is modified rather short in the former and relatively rather long in the latter in writing the 3T mark. This method is inherited to the BDs (MML/RL=1.26). In the HD-DVDs (MML/RL=1.11) in which the minimum mark length becomes short relative to a spot length, it is allowed to modify a pulse form in writing the mark while taking a blank length just after the record mark in addition to the blank length just before it into consideration.
It becomes difficult, however, for a system with further smaller MML/RL to take the conventional recording compensation method. In the super-resolution recording and reproducing system with MML/RL<1 described above, the effects of the ambient thermal environment becomes more serious.
Referring to FIGS. 11(A) to (D) showing waveforms of reflected light intensity in reproduction, the relation between the minimum mark length (MML) and the RL (resolution limit) and the effects of ambient heat will be described with taking a case of a NRZI data train in which a train of 7T marks and 7T blanks and a train of 2T marks and 2T blanks are combined. FIG. 11(A) shows a waveform after passing through an EQ (equalizer) circuit of a commercially available BD-RE (Blu-ray disc rewritable) with MML=150 nm, RL=119 nm, and a channel bit length=75 nm. All 7T marks and blanks and 2T marks and blanks are written in the same pulse conditions, and the so-called recording compensation is not carried out. In this case, the waveform is proper without missing 2T and the like even if the recording compensation is not carried out.
FIG. 11(B) shows a reproduction waveform of a medium having the same NRZI data train as above, which can carry out super-resolution reproduction in conditions of MML=75 nm, RL=119 nm, and a channel bit length=37.5 nm. MML/RL=0.63 satisfies the condition of the so-called super-resolution reproduction. As for the polarity of a write waveform, low-to-high design in which a high level is set at a mark level and a low level is set at a blank level is adopted. When all 7T marks and blanks and 2T marks and blanks are written in the same pulse conditions, the first 2T mark and the second 2T mark after a rearmost 7T space have lower reflection levels than the other marks. It has been suggested that some recording compensation is necessary.
FIG. 11(C) shows waveforms of the same NRZI data pattern by the same optical recording medium as in FIG. 11(B) without recording compensation and with the slight modification of the write conditions of the 2T marks and blanks. In FIG. 11(C), Uq indicates a waveform of not passing the EQ circuit, and Wq indicates a waveform after passing a Limit-EQ circuit. As in the case of FIG. 11(B), the reflection levels of the first and second 2T marks after 7T are low, and it is difficult to decide the level.
FIG. 11(D) shows waveforms with the exactly same conditions as FIG. 11(C) adopting a conventional recording compensation method, in which the rising edge of a write pulse length of only the first 2T mark after the 7T train is shifted forward to elongate the pulse length. The reflection level of the first 2T mark is the almost same as the third or later 2T mark levels and hence is improved. Thus, it is not allowed to elongate the write pulse anymore. The second 2T mark, however, is still hard to decide the level. This means that the 7T space affects both of the first 2T mark and the second 2T mark, and the conventional recording compensation method which refers only to a blank just before the 2T mark is not enough for high density recording.
The super-resolution recording and reproducing medium is took as an example here, but the exactly same things have been confirmed in both of the case of using the combination of the conventional medium and PRML reproduction technology and the case of shortening the minimum mark and space in the modification signal relatively to the RL than ever before.
A method is conceivable as the extension of the conventional recording compensation method, which refers to further former marks in addition to the blanks just before and just after target data in writing the target data (mark). However, since the number of combination increases, huge buffer memory and a large processing circuit become necessary, so that there are great disadvantages in processing speed and costs. Also optimizations have to be carried out for the respective combinations. Therefore, there are problems that the method is very complicated and cannot quickly respond to various external perturbations.